This invention relates to a gaseous product detection system and particularly to an improved sensing element for detecting environmental borne constituents generated as a result of combustion or the like.
Combustion detection and alarm systems employing various sensing and detecting means have been suggested, such as thermal, flame, photo-electric, ionization chamber, semi-conductors of a metal oxide or polymeric organic material, and electrolyte cell sensors. A particularly satisfactory apparatus is disclosed in the copending application of Carl F. Klein et al, Ser. No. 607,892, now U.S. Pat. No. 3,989,463, entitled "Sensing Element and Detection System Employing Such Element for Detection of Environmental Borne Material," and assigned to the same assignee as this application. The system disclosed therein is effective to sense combustion at the initial or incipient stage, as constrasted to the more advanced stage required in the prior art devices employing photo-electric, flame and thermal responses. Thus, as more fully discussed in such applications, the other devices do not react to the invisible combustion gases associated with the incipient stage. Although ionization chamber sensors, which employ radioactive alpha particles sources, capacitance sensing systems, semi-conductor sensing materials, and electrolytic cells have all been suggested to detect initial stages of combustion, they do have special constructional and operational limitations. For example, the National Technical Information Service, Springfield, Virginia 22151 has a paper identified as NASA CR-134693 which particularly discloses semi-conductive polymers which are electronegative as forming charge-transfer complexes with different gases such as in combustion being operable to produce a detectable output. A metal oxide semi-conductor probe is suggested in U.S. Pat. No. 3,603,954 and is commercially marketed. The metal oxide resistance changes to produce a detectable output in the presence of oxidizable gases such as in combustion. However, the metal oxide must be heated and oxidizable gases other than those of combustion may cause false alarms even though a dangerous condition does not exist.
The structure of the above-entitled application provides an improved sensing metal surface which can detect environmental borne products particularly at the incipient stage of combustion. However, the particular sensitive metals and the requirements of fabrication result in a somewhat costly sensor assembly. Further, the degree of sensitivity to certain combustion products may be quite low.